The present invention relates to semiconductor devices and in particular to such devices configured with multiple quantum wells.
Quantum well semiconductors and their fabrication are generally known in the art. Such devices have been prepared with alternating thin layers or structures of high charge carrier mobility and low bandgap material constituting the quantum well charge confinement regions sandwiched between barrier layers or structures of low charge carrier mobility and high bandgap material relative to each other. The quantum well charge confinement regions are sized for thicknesses less than or equal to the deBroglie wavelength for the particular charge carrier of interest. The deBroglie wavelength for a given charge carrier is given by the equation .lambda.=h/mv.sub.t where .lambda. is the deBrogl wavelength for the particular charge carrier of interest, v.sub.t is the drift velocity of that charge carrier, m is its mass, and h is Planck's constant. In addition to alternate layering of well and barrier layers, appropriately sized three dimensional quantum well structures such as cylinders or irregular-shaped solids can be located with periodic or random spacing within a layer of barrier material. The sizing and spacing of such quantum well structures is, of course, based upon the deBroglie wavelength or wavelengths of the charge carriers of interest. Molecular beam epitaxy is the means by which the ultra-thin layers or structures of the wells, and in particular applications, the barrier layers, are fabricated. Other standard semiconductor growth technologies such as, metal oxide chemical vapor deposition (MOCVD), and atomic layer epitaxy (ALE) can be used.
The prior art devices have typically been prepared for the purpose of creating or exploiting voltage or current effects dependent on charge transfer. An external DC voltage applied in a direction normal to the sandwiched charge-confining layers constituting the quantum wells can be used to rapidly move the charge carriers from one well to an adjoining well and can further be used to cause localization of charge carriers in wells. This, of course, makes such localization voltage dependent in the first instance. Doping by both invasive and non-invasive means can be used either by itself or in conjunction with the application of an external voltage to create localization of charge carriers within particular regions of the wells. Thus, pre-biasing a quantum well device and localization of charge carriers by doping are two approaches in use to predispose the bandgaps to receive particular external energy inputs which will produce device response. However, invasive and permanent introduction of semiconductor or doping structure can be expensive and by virtue of its permanency is unforgiving.
In the specific applications area of infra-red detection several additional issues arise. Infra-red detectors based on semiconductors have a band gap that falls in the infra-red range. The operation of these detectors requires excitation of an electron from the valence band to the conduction band of the semiconductor. The minimum energy required for this transition is the band gap. Thus, semiconductors cannot detect infra-red radiation which corresponds to energies below the band gap, as the technology has evolved during the past thirty years, dopants and compound semiconductors have been used to lower the infra-red detection threshold to the far infra-red. Presently, in the 8 to 12 micron window, devices based on the compound semiconductor mercury cadmium teluride (HgCdTe) are used. These devices suffer from several drawbacks. The precursor compounds used to prepare the devices by such techniques as molecular beam epitaxy (MBE) and metal-organochemical vapor deposition (MOCVD) require high temperatures, for example, greater than 350.degree. C. to dispose the molecules for deposition. This results in the constituents diffusing into the device layers where their presence is highly undesirable, for example, mercury (Hg) diffusion creates an alloy and loss of shelf life. Further, linear grading of the composition causes grading of the band gap decreasing the response of the material. Finally, constituent diffusion causes temperature instability and shortens shelf-life.
The present invention employs growth technologies such as MBE or MOCVD and conventional fabrication techniques for depositing contacts on Group III-V or II-VI semiconductor alloys. Also, the present invention employs charge localization and device response to photon impingement to produce a capacitance change proportional to the photon energy transferred.